Support both Serbian languages (Cyrillic & Latin)

[rammba]

Hi @DenverCoder1,
this PR adds a translation for Serbian latin language (sr_Latn). Also, it introduces new code for Serbian cyrillic (sr_Cyrl), which is used as a fallback for existing Serbian (sr). This fallback mechanism is based on chinese language.

Please let me know if I need to update Readme file as well.

Hey @wildbohana, since you've introduced Serbian translation in #512, fell free to add suggestions for my changes.

Description

Fixes #

Type of change

• Bug fix (added a non-breaking change which fixes an issue)
• New feature (added a non-breaking change which adds functionality)
• Updated documentation (updated the readme, templates, or other repo files)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Tested locally with a valid username
• Tested locally with an invalid username
• Ran tests with composer test
• Added or updated test cases to test new features

Checklist:

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Screenshots

[DenverCoder1]

Looks great! Thanks for the contribution 🎉